1. Field of the Invention
The invention relates to an electrically erasable and programmable memory (EEPROM, flash EPROM) with a block erasure mode. It can be applied to memory circuits and, more generally, to any integrated circuit having memory.
A mode of erasure in blocks (or block erasure mode) of this type is defined as the simultaneous erasure of the memory cells of several previously selected word lines. These selected word lines define a "block" of memorized information.
2. Description of the Prior Art
It may be recalled that there are essentially three types of erasable and programmable memories:
the EPROMs (erasable programmable memories) with a total erasure of the memory by exposure to ultraviolet rays; it is therefore not possible to erase only a zone or a particular block of the memory. PA1 the EEPROMs (electrically erasable programmable memories) with sequential electrical erasure. Depending on the organization of the memory, for example in eight-bit words, it is possible to erase an information block, sequentially, by the erasure of the word of a selected word line. Thus, to erase a four-word block, it is necessary to select the first word line, erase its eight memory cells, then do same with the second word line, and so on until the fourth word line. PA1 the flash EPROMs with sequential electrical erasure (of the entire memory). There are also flash EPROMs with sequential electrical erasure by word line, as in the EEPROMs.
The EEPROMs and flash EPROMs therefore provide for erasure by blocks, but sequentially: each word line is erased successively. This mechanism is hence very slow since, for a block of n word lines, there are n line selections and n erasures, given moreover that it is the operation of erasure that is decisive in the total time needed for the erasure of a block. For example, the selection of a line requires 250 ns and the erasure of this line requires 1 s.